This invention relates to polyacrylamide gels as used in slab gel electrophoresis.
Slab gels are particularly useful for electrophoresis in view of their ability to accommodate multiple sample analyses and the ease with which the electropherograms can be observed and read visually by identifying the locations of the various bands on the gels that correspond to the individual components. Polyacrylamide is a gel material that is widely used in slab gels.
Slab gels are frequently supplied in pre-cast form, retained between two flat transparent plates in a cassette. The plates may be glass or plastic, a common plastic being a polystyrene-acrylonitrile blend. A difficulty with certain pre-cast polyacrylamide gels is that they appear to separate from the cassette plates during storage, leaving a pathway between the gel and one or both of the plates for the sample to migrate. This apparent pathway is detrimental to the electrophoretic analysis since the solute bands in the gel tend to migrate into the pathway and spread, forming shadow bands. A shadow band is a band of protein that results from the migration of protein away from a parent protein band during electrophoresis, the parent protein band being the well-defined band that is formed as a direct result of the electrophoretic separation. Shadow bands reside mostly on the surface of the gel, whereas the parent protein bands extend relatively uniformly through the thickness of the gel. The pathways and shadow bands decrease the shelf life of precast gels and can eventually evolve into a smear of protein on the surface of the gel. The shadow bands are a particular problem in pre-cast gels that have been stored without cooling.
Another problem encountered with polyacrylamide slab gels is a tendency of the gels to stick or adhere to the plates. This presents a difficulty once the separation has been performed and the user attempts to remove the gel from the plate for purposes of staining, photographing or other observation, detection or recordation. Attempts to remove a gel that is sticking to one or both of the plates can result in a damaged gel and a ruined experiment. This problem is especially acute for gels of low concentration and for gels used for isoelectric focusing.
The polymerization reaction to form polyacrylamide is inhibited when dissolved oxygen is present in the gel-forming liquid at or near the gel plate. This is especially true when the gel plates are plastic, such as polystyrene-acrylonitrile, for example. To prevent this inhibition from occurring, a coating of polyvinylidene chloride or polyvinyl dichloride (PVDC) is often applied to the plates prior to contacting the plates with the polyacrylamide gel material. Unfortunately, these coatings produce an effect on the electrophoresis image that appears to be the result of separation between the gel and the plate. These coatings also exacerbate the sticking problem when the gel is an isoelectric focusing gel, for example one with a pH ranging from 5 to 8.